Important practical examples of diffusion-cooled lasers include radio frequency (RF) excited gas lasers and also optically pumped solid-state lasers in which the lasing medium is cooled by the heat conduction through the active lasing medium toward its externally cooled boundaries.
An RF-excited laser produces laser energy when a lasing medium within an elongated laser resonator cavity is excited by a radio frequency voltage between a pair of externally cooled electrodes. The basic transverse RF-excited gas laser was first disclosed in U.S. Pat. No. 4,169,251 to Katherine D. Laakmann ("the '251 patent"). That patent describes the laser resonator cavity as being enclosed in an elongated waveguide having two walls that are dielectrics and two walls that are the electrodes across which the RF voltage is applied.
The basic RF-excited laser evolved into high power slab gas lasers as disclosed in "Radio-Frequency Excited Stripline CO and CO.sub.2 Lasers," Gabai et al., Paper TUB4 presented at Conference on Lasers and Electro-Optics, June 1984 and U.S. Pat. No. 4,719,639 to Tulip. Such high power slab lasers enable the large volume lasing medium to be efficiently excited by an RF voltage and the resulting gas discharge and cooled by thermal conductivity through the gas to large surface area slab electrodes.
An example of a prior art slab laser 10 is shown in FIG. 1. The slab laser 10 includes first and second elongated, planar slab electrodes 12, 14 parallel to each other and extending between first and second resonator mirrors 16, 18. A gas lasing medium is positioned within an optical resonator cavity 20 formed between the slab electrodes 12, 14. When the RF voltage is applied to the gas lasing medium via the slab electrodes 12, 14, a gas discharge is formed within the resonator cavity 20 and the resonator mirrors 16, 18 form a laser beam 22 from the gas discharge within the resonator cavity 20. In the laser 10 shown in FIG. 1, the resonator mirrors 16, 18 form an unstable resonator with an exit aperture 24 such that the laser beam 22 travels laterally until the laser beam exits the resonator cavity 20 via the exit aperture. The slab electrodes 12, 14 are positioned sufficiently close to each other (e.g., 2 mm) so that resonator cavity 20 acts as a waveguide with a Fresnel number less than about 0.3 (D. Hall et al., Handbook on Molecular Lasers, P. K. Cheo, Ed., p. 170, 1987) and thereby prevent the laser beam 22 from expanding transverse to the faces of the electrodes. The slab electrodes 12, 14 act as a waveguide in that the entire laser beam 22 is reflected off of the slab electrodes multiple times along its path between resonator mirrors 16, 18.
As shown in FIG. 1, the laser beam 22 produced by the prior art slab laser 10 exits the resonator cavity 20 via the exit aperture 24. The transmission of the laser beam 22 toward and out of the exit aperture 24 can be accomplished using a negative-branch, confocal unstable resonator 26 as shown in FIG. 2 and discussed in U.S. Pat. No. 5,048,048 to Nishimae et al. The resonator mirrors 16, 18 of the resonator 26 have opposing concave reflective surfaces 28, 30 and are confocal, i.e., have a common focal point 32 on a resonator axis 34. As can be seen in FIG. 2, the exit aperture 24 is formed between the resonator walls 12, 14 by extending the first resonator mirror 16 beyond an end 36 of the second resonator mirror 18 so that the laser beam 22 is reflected by the first resonator mirror 16 out of the resonator cavity 20 through the exit aperture 24.
The typical prior art slab laser 10 shown in FIGS. 1-2 provides a laser beam of a reasonable laser beam quality, but has drawbacks. In particular, its power output is limited because of optical losses in the waveguide cavity, and because the lowest-order mode of the laser beam formed does not match the transverse gain profile of the waveguide cavity. The transverse gain profile of the prior art waveguide slab laser 10 of FIGS. 1-2 includes a relative minimum at the center of the waveguide cavity which corresponds to the position of the relative maximum of the lowest order mode of the laser beam. As a result, the lowest order mode of the laser beam achieves less than optimum power output.
In addition, the waveguide resonator cavity must be nearly perfect in order to achieve adequate laser output. In particular, the internal walls of the slab electrodes 12, 14 must be very highly polished to perform efficient waveguiding. Efficient waveguiding implies high reflection coefficients of the walls for a wide range of incident angles. This is achieved by diamond turning of bare metal electrode surfaces (A. Dutor et al., Proc. SPIE, 1996, 2773, p. 23). Further, the resonator mirrors 16, 18 must be nearly perfectly aligned with the optical axis of the resonator cavity. Such near perfection diminishes durability and adds cost to the manufacture of the waveguide lasers.
Another possibility is to position the electrodes sufficiently far apart to form a free-space laser resonator cavity with a Fresnel number of about 1.5 or more. Such a free-space resonator cavity allows the laser beam formed in the resonator cavity to expand without restriction from the resonator cavity walls. However, a laser with such a free-space resonator cavity also would have limited output power because the lowest-order mode of the resulting laser beam would not match the transverse gain profile of the resonator cavity. This results in lower electro-optical efficiency of the laser compared to lasers with a better match between the mode and gain profiles.